Circuits with noncircular shields



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CIRCUITS WITH NONCIRCULAR SHIELDS Filed June '7, 1933 4 Sheets-Sheet AATTORNEY Patented Mar. 17, 1936 UNITED STATES PATENT OFFICE CIRCUITSWITH NONCIRCULAR SHIELDS ration of New York Application June 7, 1933,Serial No. 674,764

16 Claims.

This invention is concerned with electrical transmission circuits andespecially with circuits comprising a pair of conductors surrounded byan individual shield. A particular object of the invention is to obtaina transmission circuit which has the properties of low attenuation andsubstantial freedom from interference over a wide band of frequencies.

In determining the type of transmission circuit to be used for thetransmission of high frequencies or broad bands of frequencies, thereare two important characteristics to be considered: (1) thesusceptibility of the circuit to external disturbances, such ascrosstalk from nearby circuits, and interference or noise from otheroutside sources, and (2) the high-frequency attenuation, which should bekept as low as is consistent with securing a desirable size andfavorable mechanical properties. In some applications there is a furthercharacteristic which may be of importance, namely, that the circuitshould be balanced with respect to ground.

In accordance with the present invention it is proposed to enclose apair of conductors in a conducting shield which acts to prevent externalelectromagnetic and electrostatic high-frequency disturbances fromcausing disturbances in the circuit of the pair, and conversely toprevent the currents transmitted over the pair from causing disturbancesin external circuits. Since the effectiveness of such an enclosingshield decreases with decreasing frequency, it is proposed to transmitover the pair in a balanced manner in order to reduce the effect ofinterference which may pass through the shield at low frequencies. It isalso proposed to dispose the shielded pair of conductors relatively toother similar circuits in such a manner as to reduce the interferencewhich might enter at low frequencies.

In order to reduce the high-frequency attenuation of the shieldedcircuit, it is proposed to secure low shunt losses by employing adielectric having a small power factor and to reduce the series lossesin the conductors by employing an insulating medium having a lowdielectric constant. Accordingly, it is proposed in one embodiment ofthe invention to utilize a substantially gaseous dielectric between theconductors of the pair and between these conductors and shield. Theinvention comprehends also, however, the use of non-gaseous dielectricmaterial to insulate the conductors from one another and from thesheath.

A particular object of the invention is the provision of a configurationof conductors and shield which for any given size of shield willminimize the high-frequency attenuation of the circuit. A feature of theinvention is the employment of a non-circular shield for this purpose.

More broadly, the invention is concerned with systems in which balancedpairs with individual non-circular shields are utilized for thetransmission of high-frequencies or wide bands of frequencies.

The satisfactory transmission of television images with good definitionrequires the transmission of a frequency band which may extend from zerofrequency to hundreds or perhaps thousands of kilocycles. If, forexample, it is desired to transmit, with a total of 24 reproductions persecond, an image containing 40,000 picture elements, there is required afrequency band of approximately 500 kilocycles in width. Still widerfrequency bands may be necessary for representing with adequate detailsuch scenes as a theatrical performance or an athletic event. Atelevision band of such width might be transmitted directly over ashielded pair designed in accordance with the principles of theinvention, or it might be shifted to a higher frequency position inorder to avoid the necessity of transmitting the extremely lowtelevision frequencies over the line.

Moreover, by the application of multiplexing, the wide frequency bandsobtained from a shielded pair which is designed in accordance with theinvention may be used to provide substantial numbers of narrowerfrequency bands suitable for other communication purposes, as, forexample, for telephone circuits which may require bands of about 2,500cycles in width, for high quality program circuits which may requirebands extending up to 10,000 cycles or higher, for high-speed facsimiletransmission, or for other services.

Also, it is frequently desirable in radio transmission to employ anantenna which is balanced with respect to ground rather than to transmitor receive between antenna and ground. Such, for example, is the casewhen using a diamond antenna or a horizontal dipole antenna. A balancedshielded pair of the type described herein is especially adapted forconnecting such balanced antennas with radio transmitting or receivingapparatus, inasmuch as such a pair may be designed to have lowattenuation and substantial immunity from external interference at thefrequency or frequencies employed for radio transmission.

These and other objects and features of the invention will now be morereadily understood from the following description when read inconnection with the accompanying drawings, in which Figure 1 is across-sectional diagram of a pair of conductors surrounded by a circularshield; Fig. 2 represents a cross-sectional diagram of a pair withnon-circular shield; Fig. 3 is a cross-sectional diagram of a shieldedcircuit comprising two sets of concentric conductors; Fig. 4 is a curveshowing the estimated highfrequency attenuation for a pair of wires withnon-circular shield; Fig. 5 is a graph showing the improvement inhigh-frequency resistance obtained by two specific designs of strandedconductors; Fig. 6 represents a view of a transmission structuredesigned in accordance with the invention, this structure consisting ofa pair with non-circular shield; Figs. 7 to 13 show various otherstructures embodying the principles of the invention, Figs. 14 to 16typify arrangements of apparatus which may be used in association with acircuit derived from a pair with non-circular shield; Figs. 17 and 18illustrate methods of effecting transmission over a structure withnoncircular shield; and Figs. 19 and 20 illustrate methods wherebystructures designed in accordance with the invention may be laid up incable form.

A form of construction which has been suggested as being suitable for ashielded and balanced transmission circuit is indicated by thecross-sectional diagram of Fig. 1. This structure comprises twoconductors, l and 2, which are surrounded by a circular shield 3. Such aconstruction is the subject of the copending application of E. I. Greenand H. E. Curtis, Serial No. 674,763, filed June 7, 1933 and that of E.I. Green, H. E. Curtis and S. P. Mead, Serial No. 674,762, filed June 7,1933. In these applications methods have been described forproportioning and spacing the conductors in order to obtain minimumattenuation. As indicated in Fig. 1, b1 represents the radius of eitherconductor, 01 the inner radius of the shield, and (11 the distance fromthe axis of the shield to the axis of either conductor.

Consideration of the circular form of shield in Fig. 1 shows that itapproaches fairly close to the conductors at the two sides while it isquite well removed from them at the top and bottom of the figure. Thismeans that the capacity of the circuit is greater (and the impedancecorrespondingly less) and the losses in the shield are'greater thanwould be the case if the shield could be kept at a more nearly uniformdistance from the conductors. Consequently, a lower high-frequencyattenuation might be obtained if, while keeping the total areacircumscribed by the shield unchanged, the shield were modified to anoval or elliptical form with the conductors located on the major axis ofthe oval or ellipse. Further consideration, however, indicates that astill better result might be secured with such a shape as that shown inFig. 2, where the shield consists in eifect of a semi-circular portionplaced around the outside of each conductor and two straight portionsconnecting the semi-circular portions, each conductor being coaxial withthe surrounding semicircular portion. In Fig. 2, I and 2 represent theconductors, and 3 the shield, while 122 is the radius of each conductor,dz is the distance from the center of the shield to the center of eitherconductor, and 02 is the inner radius of that portion of the shieldwhich is concentric with either conductor. To obtain a circuit which isbalanced with respect toground, the conductors l and 2 may be used oneas a return for the other. The shield 3 may be grounded if desired.

The proportioning of the arrangement of Fig. 2 in order to obtainoptimum transmission results will now be considered. It has been foundthat for many purposes the cost of a transmission circuit may beconsidered as roughly proportional to the space occupied by the circuit.This applies particularly if the circuit in question is one of a numberof circuits which are formed up into a cable. Accordingly, it isdesirable to determine the configuration of a shield of the type shownin Fig. 2 which will give minimum high-frequency attenuation for a givencross-sectional area.

This problem divides into two parts, first, the case in which theconductors are solid or are otherwise constructed in such a manner thatthe high-frequency currents travel along the surfaces of the conductors,and, second, the case in which the conductors are composed of insulatedstrands which are interwoven so as to distribute the highfrequencycurrents throughout the crosssection of the conductors. The first ofthese cases will now be taken up.

Comparing Fig. 1 and Fig. 2 it is clear that for equal areas enclosed bythe shields in the two cases It has been shown in the application ofGreen, Curtis and Mead, Serial No. 674,762, previously referred to, thatfor the configuration of Fig. 1 when using solid conductors or theirequivalent, minimum high-frequency attenuation is obtained for a giveninner diameter of shield when the value of the ratio is madeapproximately .46.

It is of interest to consider the factors which determine the value ofthis ratio. If the dielectric losses are neglected (on the assumptionthat these can be made small) the sources of loss in a shielded balancedpair are (1) the loss in the conductors, including the proximity eirectloss, and (2) the loss in the shield. These losses appear in thefamiliar expression for the attenuation in the term Where R is theresistance contribution of the conductors and the shield, and Z0 is thehighfrequency characteristic impedance of the circuit. Obviously theattenuation may be reduced either by decreasing the effective resistanceR or by increasing the impedance Z0. However. changes in theconfiguration generally affect both R and Z0, so that the problem'is oneof obtaining the balance between the different factors which givesminimum attenuation. The factors involved may be listed morespecifically as follows:

l. The increase in losses due to proximity effect (i. e., the tendencyof the high-frequency currents to avoid those parts of the conductorswhich are farthest from each other) as the spacing between conductors isreduced.

2. The increase in eddy current losses in the shield as the spacingbetween conductors is increased so as to bring them in closer proximityto the shield.

3. The change in the high-frequency characte'ristic impedance of thecircuit. This impedance reaches a maximum when .5 equals approximately.486, and decreases as the spacing is varied from that value. Since thehigh-frequency impedance varies inversely with the capacity and directlywith the inductance, the spacing of .486 represents also the conditionfor minimum capacity and maximum inductance. Those factors having nowbeen examined which determine for the configuration of Fig. 1 theconductor spacing which gives minimum high-frequency attenuation for agiven cross-sectional area, the corresponding problem in the case ofFig. 2 may be considered.

Comparison of Figs. 1 and 2 indicates that since 01 in Fig. 1 ismeasured to the center of the shield, the counterpart of the ratio forFig. 1 becomes for Fig. 2. Thus, a change in conductor spacing in Fig. 2differs from a similar change in Fig. 1 in that in the case of Fig. 2,in order to maintain a constant area within the non-circular shield, 02must be changed whenever the conductor spacing is changed.

In order to obtain further light on this problem, it is convenient nowto refer to the circuit arrangement illustrated in Fig. 3. This diagramrepresents two pairs of coaxial conductors placed side by side. Ashielded transmission circuit which is balanced to ground is obtained byusing 1 and 2 as conductors and 3 and 3', which are assumed to be incontact, as the shield. In this figure b3 designates the radius of eachconductor and c3, the radius of either half of the shield.

vA transmission circuit of the type shown in this figure is the subjectof the copending application of E. I. Green, Serial No. 674,765, filedJune 7, 1933. It is shown in that application that minimumhigh-frequency attenuation is obtained for a circuit of this kind whenthe ratio is made approximately 3.59.

Now it will be observed that Fig. 3 bears a strong resemblance to Fig.2. The similarity is emphasized if there is drawn in Fig. 2 the line aato represent an imaginary mutual plane midway between the twoconductors. Inspection of these two figures will make it clear that fora given cross-sectional area of the shield the minimum capacity (andhence maximum high-frequency characteristic impedance) will result forFig. 2 when the conductors are separated by less than twice the distance02. This is because the greater capacity between the conductor and themiddle portion of the wall aa is ofiset by the smaller capacity betweenthe conductor and those portions of the wall adjacent the shield 3. Theshape of Fig. 2 is such that the conductor spacing giving minimumcapacity and maximum impedance can be determined graphically to a verygood degree of approximation. The problem of finding minimum capacity isevidently one of balancing the change in capacity at the middle of themutual plane as the conductor spacing is changed against the change incapacity occurring at the top and bottom of the mutual plane. Bygraphical methods it may, therefore, be determined that the conditiongiving minimum capacity (also maximum highfrequency impedance andmaximum inductance) for a given cross-sectional area in Fig. 2 is thatthe conductor spacing should equal approximately .47.

One further step is required to determine the condition giving minimumhigh-frequency attenuation for a given cross-sectional area in Fig. 2.It has been seen in Fig. 1 that to obtain minimum high-frequencyattenuation, the spacing is shifted from the value of .486(corresponding to maximum impedance) to a value of .46. In Fig. 2 theshift from the spacing giving maximum impedance to that for minimumattenuation should be smaller inasmuch as the rate of change of theshield losses as the spacing ratio is changed is greater than in Fig. 1.Careful study indicates that the spacing giving minimum high-frequencyattenuation for a given cross-sectional area in Fig. 2 may be writtenwith a close degree of approximation:

On combining Equations (1) and (2), it is found that for equal areas inFigs. 1 and 2 Having determined the optimum spacing between conductorsfor Fig. 2, it is now of interest to determine the optimum size ofconductors, or, more properly speaking, the value of the ratio whichresults in minimum high-frequency attenuation for a givencross-sectional area. Comparison of Figs. 2 and 3 indicates that theoptimum value of the ratio in Fig. 2 should be fairly close to theoptimum value of 3.59 for 3 and further, that the departure should be onthe side of a slight increase in the ratio in Fig. 1 is approximately5.4. Now if 02:.6901, the corresponding ratio for Fig. 2 would beapproximately 3.75. Inspection of the two figures indicates that theoptimum ratio for Fig. 2 should not be far from this value of 3.75. Itseems,

' therefore, that for practical purposes the optimum value .of 7

may be taken as:

02 SJ (5) It is interesting to note that the optimum proportioningratios for Fig. '2 at high frequencies are independent of the frequency,the size of the conductors, and other variables. It will be obvious thatthe high-frequency attenuation of the system can be reduced byincreasing the size of the shield, keeping fixed. The size of the shieldwill ordinarily be determined by such considerations as the maximumfrequency to be transmitted over the circuit and the maximum allowableattenuation at that frequency.

It will now be of interest to determine at least approximately the valueof high-frequency attenuation which results for the circuit of Fig. 2when the values given in Equations (2) and (5) are used. It is evidentfrom an inspection of Fig. 2, which has actually been drawn using thevalues given in (2) and (5), that the capacity of a circuit with thisproportioning is very close to, but slightly greater than, the capacityof a coaxial circuit of the type shown in Fig. 3 and having E b b Thecapacity of this coaxial circuit is given by I the formula C= c abfaradsper cm. (6)

4log 2 where 'e is the dielectric constant. The capacity of the circuitof Fig. 2 with optimum propor- Insertion of numerical values indicatesthat the capacity for the circuit of Fig. 2 is about 13 per cent lessthan that for Fig. 1, Since the high-frequency characteristic impedancevaries inversely with the capacity, the circuit in Fig. 2 will have animpedance about 13 per cent higher than that of Fig. 1. Also, thehigh-frequency inductance of the circuit of Fig. 2 will be about 13 percent higher than that of Fig. 1.

With regard to the resistance of the circuit of Fig. 2, a comparisonwith Fig. 1 indicates that the losses introduced by the sheath will beless than in Fig. 1 because of the greater separation of the conductorsfrom the sheath. The proximity effect in Fig. 1 for the case of solidconductors has been shown to be small owing to the reaction of thesheath currents. In Fig. 2 a larger proximity effect may be anticipated.It is probably on the conservative side to assume that the resistancereduction due to lower sheath losses is balanced by the resistanceincrease due to proximity effect, and that the resistances of the twocircuits are equal. On this basis the high-frequency attenuation of thecircuit of Fig. 2 would be lower than that of Fig. 1 by the amount ofthe reduction in capacity, i. e, about 12 per cent.

Fig. 4 shows a curve for the high-frequency attenuations of the circuitsof Figs. 1 and 2 on this basis, derived on the assumption that the innerdiameter of shield in Fig. 1 is one-half inch and that the two circuitsoccupy the same cross-sectional area. It will be noted that at afrequency of 1,000 kilocycles the attenuation of the pair withnon-circular shield is approximately 3.6 db per mile. Hence, ifrepeaters having a gain of 60 db each were connected in the circuit,these could be spaced at intervals of about 17 miles.

The case where the conductors in Fig. 2 are composed of insulatedstrands will now be considered. As will be pointed out later, suchstranding may be accomplished in various ways. Ordinarily the purpose ofstranding would be to reduce the resistance of the conductors at highfrequencies by counteracting the tendency of the currents to concentrateon the surface of the conductor, and to increase the internal inductanceof each conductor. Both of these results tend to decrease thehigh-frequency attenuation. stranding may also be advantageous from thestandpoint of obtaining a flexible structure.

In order to counteract the tendency of the currents to concentrate onthe surface of the conductor at high frequencies it is essential thatthe insulated strands be passed back and forth toward and from thecenter of the conductor. With a suitable method of stranding thehighfrequency current may be distributed substantially uniformly overthe cross-section of the conductor.

The high-frequency resistance of the one stranded conductor alone (inabohms per centimeter) may be written as E; K 172 being the radius ofthe conductor, 1 the frequency in cycles, A the conductivity(approximately 5.8x abmhos per centimeter cube for copper), and n theratio of the resistance of the stranded conductor to the resistance of asolid conductor of the same diameter at the frequency f.

The value of n for a conductor that is stranded in such a manner thatthe current density is uniform throughout its entire cross-section canbe obtained from a formula by S. Butterworth, published in thePhilosophical Transactions of the Royal Society of London, vol. 222, p.57.

Equation (85) therein should be modified by the omission of the twoterms which involve D when it will read by the expression l fl b 6 Thevalue of n maybe determined by dividing R bv R1.

Fig. 5 shows how the value of it varies with frequency for two assumedconditions of strandlng. It will be observed that the value of n may bemade considerably less than unity at frequencies in the vicinity of 500kilocycles or above. By the use of Formula (8) the stranded conductorsmay be designed so as to have as low a value of n as practicable at themaximum frequency to be transmitted over the circuit.

As pointed out below, it would be possible, instead of filling up thecomplete conductor crosssection with insulated strands, to arrange thestrands in an annular cross-section, the stranding being carried out insuch a way that the path of any individual strand would extend betweenthe outer and inner circumferences of the annulus. If the conductors arestranded in this or some other manner the value of n may be determinedby computation or experiment.

In the application of Green and Curtis, Serial No. 674,763, it is shownthat when the conductors in Fig. 1 are completely stranded, for thevalues of n ordinarily realizable in practice, the minimumhigh-frequency attenuation will be obtained when the spacing ratio isgiven a value of from .41 to .42 and the ratio of radii approximately5.0. Comparison of Figs. 1 and 2 indicates that inasmuch as the lossesin the shield are lower in Fig. 2, the spacing ratio in Fig. 2 should beslightly larger than in Fig. 1. It is clear, however, that this ratioshould be closer to the value of .41 or .42 than to the value of .47which gives maximum circuit impedance for Fig. 2. A reasonable range ofworking values for different values of 11 would appear to be from .42 to.43.

As in the case of solid conductors, the optimum ratio of radii for thecase of stranded conductors can best be determined by comparison ofFigs. 2 and 3. In the co-pending application of E. I. Green, Serial No.674,765, it is shown that when the conductors in Fig. 3 are stranded,the ratio of radii which gives minimum attenuation 'ranges from about3.3 to 4.3, depending upon the value of n.

In view of the close resemblance of Figs. 2 and 3, the ratio for Fig. 2should evidently be very close to this range of values. Another check onthe optimum I ratio is obtained by comparison with Fig. 1, for

which the ratio should be approximately 5.0. The corresponding value ofthe ratio in Fig. 2 would be approximately 3.6. It is not necessary todetermine the optimum value of with a high degree of accuracy inasmuchas, for reasonable departures from the optimum, the attenuation will notbe appreciably increased. For the values of n obtained in practice,results closely approaching the optimum will be obtained in the range ofvalues t is now of interest to compare the attenuation of the circuit ofFig. 2 when using stranded conductors with that of Fig. 1 for strandedconductors. Considerations similar to those which have been set forthfor the case of solid conductors make it evident that the capacity ofthe circuit of Fig. 2 with stranded conductors should be about 12 percent less than that of Fig. 1 and the inductance and high-frequencyimpedance about 13 per cent greater than for Fig. 1.

Since the stranding of the conductors eliminates proximity effect, thereremains only the sheath loss to be added to the resistance of theconductors themselves. Inasmuch as the sheath losses are lower in Fig.2, the resistance of the circuit of Fig. 2 will be lower than for Fig.1, the actual amount depending on the value of n. For ordinary workingvalues of n it is thought that the resistance of Fig. 2 with strandedconductors may be of the order of to 15 per cent less than that of Fig.1 for stranded conductors.

It follows from the above that the high-frequency attenuation of thecircuit of Fig. 2 with stranded conductors will be of the order of to 30per cent lower than that of the Fig. 1 circuit with stranded conductors,the actual difference depending upon the nature of the strand- Theforegoing derivation of the proportioning of a circuit with non-circularshield in order to obtain minimum high frequency attenuation has largelybeen directed toward the cases where the insulating medium is largelygaseous so that the dielectric constant is substantially unity and aleakage conductance substantially zero. It can be shown, however, thatthe optimum proportioning will remain substantially unchanged for othertypes of dielectric. Thus if the space between conductors and shield isfilled with a homogeneous non-gaseous dielectric as, for example, rubberor oil, the ratios giving minimum high-frequency attenuation should bethe same as for a gaseous dielectric. This will also be the case when amixture of dielectrics is employed, for example, a combination ofgaseous and non-gaseous dielectrics provided that the arrangement of thedielectric is such as not to distort the path which would be assumed bythe dielectric flux if the dielectric medium were entirely gaseous.Where a combination of dielectrics is employed in such a manner as toproduce such distortion of the flux both the ratios for optimumproportioning may be changed to some extent, but, in general,characteristics approaching the optimum will be obtained for the valueswhich have previously been set forth.

Some of the fundamental principles of the invention having now been setforth, consideration may be given to types of structures in which theseprinciples may be incorporated. Fig. 6 represents a view of atransmission structure consisting of a pair with non-circular shield. Inthis figure, l and 2 represent two solid conductors which are held inposition with respect to one another and the non-circular shield 3 byinsulating spacers 4 or other suitable devices. The conductors of thepair are connected one as a return for the other, as is indicatedconventionally by the generator G. If desired, the shield 3 may begrounded as. indicated on the drawings.

The conductors l and 2 may be of such a type that currents offrequencies well above the andible range travel substantially on theouter surfaces of the conductors. For example, the conductors may besolid wires or may be tubular. If tubular conductors are employed, theirwall thickness will ordinarily depend upon mechanical rather thanelectrical considerations, since only a very thin wall is required.Also, the conductors may consist of a cylindrical assembly of conductingstrips, tapes, ribbons, wires or the like which are not insulated fromone another. Such a form of construction might be particularly desirablewhere a flexible structure is required. One construction of this type isindicated in Fig. '7, the conductors l and 2 in this case being composedof uninsulated wires stranded together. As has already been pointed out,it may be found advantageous to construct the conductors l and 2 of anumber of strands, filaments, tapes or the like which are insulated fromone another and are interwoven or braided together in any of Variousways. In this manner there may be obtained a reduction in thehigh-frequency resistance of the conductors and an increase in theirinternal inductance, both of which tend to reduce the high-frequencyattenuation of the circuit. In order to obtain these results it isessential that the insulated strands be passed back and forth toward andfrom the center of the conductor.

With a suitable method of stranding, the highfrequency current may bedistributed substantially uniformly over the cross-section of theconductor. One method of securing this result is to strand the conductorin a manner similar to that used in the manufacture of rope. Thus,several individual strands (for "example, three) would first be twistedtogether; next several of these groups would be twisted together to formlarger groups and several of the larger groups would be twistedtogether, the process being continued until the desired total number ofstrands is obtained. If the stranding interval or pitch is madedifferent for the successive twisting operations, it will be found thatwith such a method any one strand in going along the conductor travels apath back and forth between the center of the conductor and itsperiphery. A structure employing conductors stranded in this manner isillustrated in Fig. 8.

Instead of being twisted together as described above, the strands mightbe interwoven or braided in other ways so as to produce the desiredeffect. Also, it would be possible, as already noted, to employ anannular cross-section for the insulated strands, the core of theconductor being filled up with some non-conducting material such asjute, or with a conducting material such as copper or steel to providestrength or rigidity. The stranding might preferably be designed in sucha way that the path of any strand would extendbetween the outer andinner circumferences of the annulus. A structure employing strandedconductors having an annular'cross-section of this kind is illustratedin Fig. 9.

Any of various forms or shapes may be employed for the insulationbetween the two conductors. and between conductors and sheath. Onepossible arrangement would be to use a continuous spirally appliedstring or strip of dielectric material around each conductor and anotherspirally applied string to separate them from the shield. An arrangementof this type is illustrated in Fig. 10, where 5 and B are strings ofinsulating material wrapped spirally around the conductors, l and 8 arethin strips of insulating material covering the conductors and thespirally wrapped strings and 9 is a spirally applied string separatingtheconductors from the shield. Generally, it will be desirable that theamount of insulating material be a minimum in order that the dielectricbetween the two conductors may be largely, gaseous. In some cases,however, it will be found advantageous to use a dielectric which iswholly or partly non-gaseous, as, for example, rubber insulation. Astructure with a dielectric of this kind is shown in Fig. 11. For theinsulation arrangements that would ordinarily be employed in practice,the optimum configuration of the circuit will be approximately the sameas for the assumed condition of a gaseous dielectric.

The shield surrounding the two conductors, instead of being formed of asingle tube, might consist of a cylindrical assembly of conductingstrips, tapes, wires, ribbons or the like. Such forms of constructionmight be particularly advantageous where a flexible structure isdesired. One construction of this kind is illustrated in Fig. 12, wherethe shield consists of a number of spiral segments formed into a tube.If desired the shield may be surrounded by a water-proof sheath orcovering lfl, which may be composed of lead, rubber or other suitablematerial.

In connection with the shield, it may be noted that, in addition toperforming an electrical function by protecting against inductiveeffects, it may be useful in affording mechanical protection to thecircuit and thereby permitting the use, to a very considerable extent,of an air dielectric. Due to skin effect the high-frequency currentswill penetrate only a little way into the shield, so that the electricalrequirements are satisfied by a very thin shield. Consequently, thethickness of the shield will ordinarily be determined by mechanicalconsiderations and will usually be such that it does not enter into theproblem of determining the optimum configuration of conductors andshield.

The use of the shield will ordinarily make it possible where desired toallow the signals transmitted over the pair to drop down to a minimumlevel. determined by the noise due to thermal agitation of electricityin the conductors. Hence, the shield facilitates the spacing ofintermediate amplifiers in the circuit at wider intervals than wouldotherwise be possible.

It has been proposed in connection with the circuitof Fig. 1 that theconductors be transposed at frequent intervals in order to reduce thepossibility of. interference into or from the circuit at low frequencieswhere the shield is less efiective. Such transposition can readily beaccomplished for the circuit of Fig. 1 by twisting the two conductorshelically about the axis of the shield. It is evident that atransposition arrangement. of this kind is not suitable for a structureof the type represented in Fig. 2, since the twisting of the conductorswith respect to the shield would bring them in contact with or in tooclose proximity to the shield. However, if it should be desired totranspose the circuit of Fig. 2 in order to improve its characteristicsat low frequencies, this can be done by the method illustrated in Fig.13, where the entire structure consisting of conductors and non-circularshield is twisted about its axis. Such twisting has, of course, thepossible disadvantage of increasing the space which may be occupied bythe structure if it is to be laid up in a cable with other structures.

The structures which have been illustrated in Figs. 6 to 13, comprisingconductors surrounded by non-circular shields, may be employed astransmission media for various types of transmission systems. Some ofthe systems which may be used in this manner are illustratedschematically in Figs. 14 to 16.

Fig. 14 is a diagram of a multiplex carrier telephone system includingthe channel modulating and demodulating equipment, the filter apparatusrequired for segregating the different channels and the amplifyingapparatus at the terminals and at intermediate points along the line. Inthis figure voice-frequency currents derived from the instruments SS areapplied to individual modulators as indicated by CM which convert themto carrier frequencies. The wanted sidebands are selected by channelfilters CF and may, after passing through the amplifier TA, be appliedto the line section LC comprising a pair of wires with non-circularshield designed in accordance with the invention. At suitable points inthe line repeaters such as IR may be inserted. At the receiving end theincoming carrier channels may, after being amplified in the receivingampliler RA, be separated by means of the channel filters SF and bebrought again to voice frequencies in channel demodulators as indicatedby CD. The arrangement as shown serves for transmission in one directionand a duplicate arrangement would be provided for the opposite directionof transmission.

Fig. 15 is a diagram of a television system in which the line circuit isprovided by a pair of conductors having a non-circular shield. In thisdiagram TT represents the television transmitting apparatus by means ofwhich the television signals are applied to the line circuit LC. The

- transmitting apparatus may be such as to furnish to the line a band offrequencies extending from approximately zero frequency to a highfrequency determined by the degree of image definition which it isdesired to obtain. If desired, however, this apparatus may also includemodulating equipment whereby the television band of signals is shiftedto a higher position in the frequency spectrum. At the receiving end thetelevision receiving apparatus TR takes the band of signals delivered bythe line and converts it into the desired image, this apparatusincluding whatever demodulating apparatus may be required to shift thefrequency position of the television band in a manner reverse to thatemployed at the transmitting end. The arrangement illustrated serves fora single direction of transmission and may be duplicated for theopposite direction of transmission. It is obvious that other signals as,for example, those from voice channels, may be combined with thetelevision signals for transmission over the line.

Fig. 16 is a diagram of a radio transmitting system in which theconnection from the transmitting apparatus to the transmitting antennais secured bymeans of a pair with non-circular.

shield and the connection between the receiving antenna and thereceiving apparatus is similarly obtained. In this diagram RT designatesradio transmitting apparatus and TL a transmission circuit forconnecting this apparatus to the transmitting antenna TA, while RAdesignates a receiving antenna whose output is transmitted over thereceiving circuit RL to the radio receiving apparatus RR.

The terminal apparatus and amplifiers which may be used in connectionwith a transmission line such as previously described may be shieldedfrom electrical interference from outside sources by surrounding themwith sheet metal compartments. These compartments may be connected tothe shield of the transmission line if desired. Such compartments areillustrated in Figs. 14, 15 and 16.

The arrangements thus far described have contemplated the use of thestructure with non-circular shield for providing a balanced transmissioncircuit in which the conductors I and 2 are employed one as a return forthe other. If desired, it would be possible to derive from. the samestructure an independent transmission circuit by connecting theconductors I and 2 in parallel and using the sheath as a return. Figs.17 and 18 illustrate two different methods of deriving two independenttransmission circuits, one balanced and one unbalanced, from thestructure of Fig. 2. In Fig. 1'7, the generator G1 is connected to theconductors I and 2 through a transformer T1, thus providing a balancedto ground circuit. Generator G2 is connected between the electricalmidpoint of the conductors I and 2, provided by a center tap on thesecondary of transformer T1 and the shield, providing a second circuit,the latter being unbalanced to ground. In Fig. 18, the generator G1 isconnected directly to the conductors I and 2 providing a balanced toground circuit. An unbalanced to ground circuit is obtained byconnecting a generator G2 between the shield and the midpoint of aresistance R shunted across the generator G1.

Figs. 19 and 2-0 illustrate methods in which structures of the typeshown in Figs. 2 to 11 may be laid up in cable form. In the lay-upsillustrated the various circuits are so disposed with reference to oneanother as to tend to minimize the coupling which would exist betweenthem at low frequencies where the shielding is only partially effective.

It will be noted in Figs. 19 and 20 that the oval shields do not extendto the center of the cable. This center space may be left empty or maybe filled with jute, paper or other similar material. It may be utilizedby filling it with pairs of wires or by placing a coaxial conductor init. In Figs. 19 and 20 a coaxial conductor is shown in the center of thecable, II and I2 being the inner and outer conductors, respectively.

It will be obvious that the general principles disclosed herein may beincorporated in many other organizations different from thoseillustrated without departing from the spirit of the invention asdefined in the following claims.

What is claimed is:

1. An electrical transmission circuit comprising two cylindricalconductors, one of said conductors being connected as a return for theother to form a high frequency transmission path, a conducting shieldsurrounding said conductors, said shield comprising two portions ofsemi-circular cross-section with flat portions joining the ends of said.semi-circular portions and tangent lar portion of said shield, theinteraxial separation of said conductors being less than the diameter ofsaid semi-circular portions, said conductors and shield being insulatedfrom one another, the transmission path formed from said cylindricalconductors acting one. as a return for the other having connectedthereto apparatus for applying thereto and receiving and utilizingtherefrom a band of signal frequencies whose range is many times that ofthe audible range, said path with its associated shield acting totransmit without excessive attenuation the band of frequencies soapplied.

2. An electrical transmission circuit comprising two cylindricalconductors, one of said conductors being connected as a return for theother to form a high frequency transmission path, a conducting shieldsurrounding said conductors, said shield comprising two portions of,

semi-circular cross-section with flat portions joining the ends of saidsemi-circular portions and tangent thereto and so disposed that each ofvsaid conductors is surrounded coaxially by' a semicircular portion ofsaid shield, the interaxial separation of said conductors being lessthan the diameter of said semi-circular portions, said conductors andshield being insulated from one another by a substantially gaseousdielectric, the transmission path formed from said cylindricalconductors acting one as a return for the other having connected theretoapparatus for applying thereto and receiving and utilizing therefrom aband of signal rfequencies whose range is many times that of the audiblerange, said path with its associated shield acting to transmit withoutexcessive attenuation the band of frequencies so applied.

3. An electrical transmission circuit comprising two cylindricalconductors, one of said conductors being connected as a return for theother to form a high frequency transmission path, a conducting shieldsurrounding said conductors, said shield comprising two portions ofsemi-circular cross-section with flat portions joining the ends of saidsemi-circular portions and tangent.

thereto and so disposed that each of said conductors is surroundedcoaxially by a semi-circular portion of said shield, the interaxialseparation of said conductors being less than the diameter of saidsemi-circular portions, said conductors and shield being insulated fro-mone another, said conductors being of such a type that conduction ofcurrents whose frequencies are substantially above the audible rangetakes place substantial ly on the surface of said conductors, the.transmission path formed from said cylindrical conductors acting one asa return. for the other having connected thereto apparatus for applyingthereto and receiving and utilizing therefrom a band of signalfrequencies whose range is many times that of the audible range, saidpath with its associated shield acting to transmit. without excessiveattenuation the band of frequencies so applied. 7

4. An electrical transmission circuit comprising two cylindricalconductors, one of said conductors being connectedv as a return for theother to form a high frequency transmission path, a

conducting shield surrounding said conductors, said shield comprisingvtwo portions of semicircular crosssection withv flat portionsv joiningthe ends of. said semi-circular portions: and tan.- gent thereto and sodisposed that each of said.

conductors:iszsurroundedcoaxially by a semi-circular portion, of saidshield, the interaxial separation. of said conductors being less thanthe diameter of said semi-circular portions, said conductors and shieldbeing insulated from one another, each of said conductors consisting ofa plurality of conducting strands insulated from one another, the.transmission path formed from said cylindrical conductors acting one asa return for the other having connected thereto apparatus for applyingthereto and receiving and utilizing therefrom a band of signalfrequencies whose range is many times that of the audible range, saidpath with its associated shield acting to transmit without excessiveattenuation the band of frequencies so applied.

5. An electrical transmission circuit comprising two cylindricalconductors, one of said conductors being connected as a return for theother, a flattened conducting shield surrounding said conductors, saidshield comprising two portions of semi-circular cross-section with flatportions joining the ends of said semi-circular portions and tangentthereto and so disposed that each of said conductors is surroundedcoaxially by a semi-circular portion of said shield, said conductors andshield being insulated from one another; the ratio of the interaxialseparation of said conductors to the inner diameter of eithersemi-circular portion of said shield plus the length of either flatportion of said shield being less than the ratio which gives maximumcharacteristic impedance of the circuit, and approximately equal towhere, in a system of cylindrical conductors surrounded by a circularshield and designed for minimum attenuation, 2111 is the separationbetween conductors and 01 is the radius of the shield; and the ratio ofthe inner diameter of either semi-circular portion of said shield to theouter diameter of each of said conductors being of the order ofmagnitude of where, in, a system of cylindrical conductors eachsurrounded by its own individual shield and designed for minimumattenuation, as is the radius of the shield and D3 is the radius of theconductor; and the. high frequency attenuation of the circuit having aflattened shield being a minimum for the cross-sectional area includedwithin said shield. V

6. An. electrical transmission circuit comprising two cylindricalconductors, one of said conductors being connected as a return for theother, a flattened conducting shield surrounding said conductors, saidshield comprising two portions of semi-circular cross-section with flatportions joining the ends of said semi-circular portions and tangentthereto and so disposed that each of said conductors is surroundedcoaxially by a semi-circular portion of said shield, said conductors andshield being insulated from one another by a substantially gaseousdielectric; the ratio of the interaxial separation of said conductors tothe inner diameter of either semi-circular portion of said shield plusthe length of either flat portion of said shield being less than theratio which gives maximum characteristic impedance of the circuit, andapproximately equal to where, in a system of cylindrical conductorssurrounded by a circular shield and designed for minimum attenuation,2111 is the separation between conductors and 01 is the radius of theshield; and the ratio of the inner diameter of either semi-circularportion of said shield to the outer diameter of each of said conductorsbeing of the order of magnitude of where, in a system of cylindricalconductors each surrounded by its own individual shield and de signedfor minimum attenuation, as is the radius of the shield and b: is theradius of the conductor; and the high-frequency attenuation of thecircuit having a flattened shield being a minimum for thecross-sectional area included within said shield.

'7. An electrical transmission circuit comprising two cylindricalconductors, one of said conductors being connected as a return for theother, said conductors being of such a type that conduction of currentswhose frequencies are substantiallly above the audible range takes placesubstantially on the surface of said conductors, a flattened conductingshield surrounding said conductors, said shield comprising two portionsof semi-circular cross-section with flat portions joining the ends ofsaid semi-circular portions and tangent thereto and so disposed thateach of said conductors is surrounded coaxially by a semi-circularportion of said shield, said conductors and shield being insulated fromone another; the ratio of the interaxial separation of said conductorsto the inner diameter of either semi-circular portion of said shieldplus the length of either flat portion of said shield being less thanthe ratio which gives maximum characteristic impedance of the circuit,and approximately equal to where, in a system of cylindrical conductorssurrounded by a circular shield and designed for minimum attenuation,2dr is the separation between co-nductors and cr is the radius of theshield; and the ratio of the inner diameter of either semi-circularportion of said shield to the outer diameter of each of said conductorsbeing slightly larger than where, in a system of cylindrical conductorseach surrounded by its own individual shield and designed for minimumattenuation, 03 is the radius of the shield and 173 is the radius of theconductor; and the high frequency attenuation of the circuit having theflattened shield being a minimum for the cross-sectional area includedwithin said shield.

8. An electrical transmission circuit comprising two cylindricalconductors, one of said conductors being connected as a return for theother, each of said conductors consisting of a plurality of conductingstrands insulated from one another, a flattened conducting shieldsurrounding said conductors, said shield comprising two portions ofsemi-circular cross-section with flat portions joining the ends of saidsemi-circular portions and tangent thereto and so disposed that each ofsaid conductors is surrounded coaxially by a semi-circular portion ofsaid shield, said conductors and shield being insulated from oneanother; the ratio of the interaxial separation of said conductors tothe inner diameter of either semi-circular portion of said shield plusthe length of each flat portion of said shield being less than the ratiowhich gives maximum characteristic impedance of the circuit, andapproximately equal to where, in a system of cylindrical conductorssurrounded by a circular shield and designed for minimum attenuation,2111 is the separation between conductors and 01 is the radius of theshield; and the ratio of the inner diameter of either semi-circularportion of said shield to the outer diameter of each of said conductorsbeing of the order of magnitude of 3 e where, in a system of cylindricalconductors each surrounded by its own individual shield and designed forminimum attenuation, 03 is the radius of the shield and ha is the radiusof the conductor; and the high frequency attenuation of the circuit withthe flattened shield being a minimum for the cross-sectional areaincluded within said shield.

9. An electrical transmission circuit comprising two cylindricalconductors, one of said conductors being connected as a return for theother, a conducting shield surrounding said conductors, said shieldcomprising two portions of semi-circular cross-section, and flatportions joining the ends of said semi-circular portions and tangentthereto and so disposed that each of said conductors is surroundedcoaxially by a semi-circular portion of said shield, said conductors andshield being insulated from one another, the ratio of the interaxialseparation of said conductors to the inner diameter of eithersemi-circular portion of said shield plus the length of either flatportion of said shield, being approximately in the range between .42 and.46 and the ratio of the inner diameter of either semi-circular portionof said shield to the outer diameter of each of said conductors beingapproximately in the range between 3.3 and 4.3.

10. An electrical transmission circuit comprising two cylindricalconductors, one of said conductors being connected as a return for theother, said conductors being of such a type that conduction of currentswhose frequencies are substantially above the audible range takes placesubstantially on the surface of said conductors, a conducting shieldsurrounding said conductors, said shield comprising two portions ofsemi-circular cross-section, and flat portions joining the ends of saidsemi-circular portions and tangent thereto and so disposed that each ofsaid conductors is surrounded coaxially by a semi-circular portion ofsaid shield, said conductors and shield being insulated from oneanother, the ratio of the interaxial separation of said conductors tothe inner diameter of either semi-circular portion of said shield plusthe length of either flat portion of said shield being approximately .46and the ratio of the inner diameter of either semi-circular portion ofsaid shield to the outer diameter of each of said conductors beingapproximately 3.7.

11. An electrical transmission circuit comprising two cylindricalconductors, one of said conductors being connected as a return for theother, each of said conductors consisting of a plurality of conductingstrands insulated from one another, a conducting shield surrounding saidconductors, said shield comprising two portions of semi-circularcross-section, and flat portions joining the ends of said semi-circularportions and tangent thereto and so disposed that each of saidconductors is surrounded coaxially by a semi-circular portion of saidshield, said conductors and shield being insulated from one an other,the ratio of the interaxial separation of said conductors to the innerdiameter of either semi-circular portion of said shield plus the lengthof either flat portion of said shield being approximately in the rangebetween .42 and .43 and the ratio of the inner diameter of eithersemi-circular portion of said shield to the outer diameter of each ofsaid conductors being approximately in the range between 3.3 and 4.3.

12. An electrical transmission circuit comprising twocylindricalconductors, one of said conductors being connected as a return for theother, said conductors being of such a type that conduction of currentswhose frequencies are substantially above the audible range takes placesubstantially on the surface of said conductors, a conducting shieldsurrounding said conductors, said shield comprising two portions ofsemi-circular cross-section, and flat portions joining the ends of saidsemi-circular portions and tangent thereto and so disposed that each ofsaid conductors is surrounded coaxially by a semi-circular portion ofsaid shield, said conductors and shield being insulated from one anotherby a substantially gaseous dielectric, the ratio of the interaxialseparation of said conductors to the inner diameter of eithersemi-circular portion of said shield plus the length of either fiatportion of said shield being approximately .46 and the ratio of theinner diameter of either semicircular portion of said shield to theouter diameter of each of said conductors being approximately 3.7.

13. An electrical transmission circuit comprising two cylindricalconductors, one of said conductors being connected as a return for theother, each of said conductors consisting of a plurality of conductingstrands insulated from one another, a conducting shield surrounding saidconductors, said shield comprising two portions of semi-circularcross-section, and flat portions joining the ends of said semi-circularportions and tangent thereto and so disposed that each of saidconductors is surrounded coaxially by a semi-circular portion of saidshield, said conductors and shield being insulated from one another by asubstantially gaseous dielectric, the ratio of the interaxial separationof said conductors to the inner diameter of either semi-circular portionof said shield plus the length of either fiat portion of said shieldbeing approximately in the range between .42 and .43 and the ratio ofthe inner diameter of either semi-circular portion of the shield to theouter diameter of each of said conductors being approximately in therange between 3.3 and 4.3.

14. An electrical transmission structure comprising a pair ofcylindrical conductors, one of said conductors being connected as areturn for the other, a conducting shield surrounding said conductor,said shield comprising two portions of semi-circular cross-section withfiat portions joining the ends of said semi-circular portions andtangent thereto and so disposed that each of said conductors issurrounded coaxially by a semicircular portion of said shield, saidconductors and shield being insulated from one another, means forconnecting said conductors in series to form a high frequencytransmission circuit, the transmission path formed from said cylindricalconductors acting one as a return for the other having connected theretoapparatus for applying thereto and receiving and utilizing therefrom aband of signal frequencies whose range is many times that of the audiblerange, said path with its associated shield acting to transmit withoutexcessive attenuation the band of frequencies s0 applied, and means forconhecting to the electrical center of said pair of conductors forestablishing an independent high frequency transmission circuit betweensaid pair of conductors in parallel as one conductor and said shield asthe other conductor.

15. An electrical transmission structure comprising a pair ofcylindrical conductors, one of said conductors being connected as areturn for the other, a conducting shield surrounding said conductors,said shield comprising two portions of semi-circular cross-section withflat portions joining the ends of said semi-circular portions andtangent thereto and so disposed that each of said conductors issurrounded coaxially by a semi-circular portion of said shield, saidconductors and shield being insulated from one another, means forconnecting said conductors in series to form a balanced to ground highfrequency transmission circuit, the transmission path formed from saidcylindrical conductors acting one as a return for the other havingconnected thereto apparatus for applying thereto and receiving andutilizing therefrom a band of signal frequencies whose range is manytimes that of the audible range, said path with its associated shieldacting to transmit without excessive attenuation the band of frequenciesso applied, and means for connecting said pair of conductors in parallelfor establishing an unbalanced to ground high frequency transmissioncircuit between said pair of conductors and said shield.

16. In an electrical transmission system, a line circuit comprising twocylindrical conductors, one of said conductors being connected as areturn for the other to form a high frequency transission path, aconducting shield surrounding said conductors, said shield andconductors being insulated from one another, said shield comprising twoportions of semi-circular cross-section with flat portions joining theends of said semi-circular portions and tangent thereto and so disposedthat each of said conductors is surrounded coaxially by a semi-circularportion of said shield, the transmission path formed from saidcylindrical conductors acting one as a return for the other havingconnected thereto apparatus for applying thereto and receiving andutilizing therefrom a band of signal frequencies whose range is manytimes that of the audible range, said path with its associated shieldacting to transmit without excessive attenuation the band of frequenciesso applied, and repeaters at intermediate points in said line circuitfor amplifying the range of frequencies transmitted.

ES'ITLL I. GREEN. FRANK A. LEIBE.

